Rat embryo fibroblasts have been shown to have receptors for MSA (multiplication stimulating activity), to respond to MSA (DNA synthesis), and to produce MSA the concentration of MSA in medium conditioned by the rat embryo fibroblasts approaches the concentration of MSA in fetal rat serum. Fibroblasts from skin and kidney of the pygmy mouse (pg/pg) have been shown to have receptors for MSA with characteristics indistinguishable from MSA receptors on fibroblasts from normal littermates.